1. Filed of the Invention
The present invention relates to a DC motor apparatus, and more particularly, to & DC motor which can be started with a small starting current.
2. Prior Art
In a conventional DC motor which requires a large driving force, a magnetic path is formed by a magnetic material section to enhance a magnetic flux density generated by a coil. A magnetic flux from a permanent magnet incorporated in a movable part of a motor converges in such a magnetic material section having high permeability. Therefore, since an intensive magnetic attracting force is generated between the permanent magnet and the magnetic material section it is necessary to supply a large exciting current from a power supply to such a DC motor.
FIG. 1 is a cross-sectional view indicating magnetic pole portions of a rotor and a stator of an ordinary DC motor. A permanent magnet 1 of the rotor has magnetic poles arranged alternately and coils 2 to 5 of the stator are formed around iron core 8. As explained above, since a magnetic flux generated from permanent magnet 1 converges in iron core 6 consisting of a magnetic material having a high permeability, an intensive magnetic attracting force is generated between iron core 6 and permanent magnet 1 and the rotor is restricted to stay in a constant direction. Therefore, it is necessary to generate a magnetic flux sufficient to overcome such restricting forces generated by other coils (coils 2 and 4 in FIG. 1), in order to rotate the rotor.
FIG. 2 illustrates an example of an ordinary structure of a conventional DC motor apparatus. An output signal of position detector 7 for detecting a position of the permanent magnet of the rotor is supplied to distributor 8. Distributor 8 switches a current path to selectively supply a current from a DC power supply to any one of coils 2 to 5. When permanent magnet 1 begins to revolve, the current paths which supply currents to coils 2 to 5 are sequentially switched and the rotor of the motor comprising permanent magnet 1 rotates.
FIG. 3 illustrates waveforms of the output of position detector 7 in the DC apparatus shown in FIG. 2 and waveforms of currents flowing through coils 2 to 5. In the structure shown in FIG. 2, in the case where permanent magnet 1 of the rotor is rotating, outputs a and b of position detector 7 alternately go to a positive level periodically due to change in position of the magnetic poles of permanent magnet 1. When output a at a positive level and output b at a zero level are input to distributor 8, positive currents flow through coils 2 and 4 and negative currents flow through coils 3 and 5. Next, when the position of the magnetic poles change and outputs a and b supply signals at positive and zero levels, respectively, to distributor 8, negative currents flow through coils 2 and 4 and positive currents flow through coils 3 and 5. Thus, the switching of coils currents is repeated due to the switching of signals output from position detector 7.
In the conventional DC motor structured as illustrated in FIG. 1 and FIG. 2, an intensive magnetic attracting force is generated between iron core 6 and permanent magnet 1, and thus the rotor of the motor is restricted to stay in a constant direction. Therefore, a large current is required to start such a DC motor and DC power supply source 9 and distributor 8 are both required to have large current capacities. FIG. 4 illustrates waveforms of the output of position detector 7 and waveforms of currents flowing through coils 2 to 5 for start. In the structure shown in FIG. 2, when permanent magnet 1 of the rotor stops, if outputs a and b of position detector 7 are at positive and zero levels, respectively, distributor 8 receiving outputs a and b causes positive continuous currents to flow through coils 2 and 4 and negative continuous currents to flow through coils 3 and 5.